It is known that the cross linking of polymer plastic material is enhanced and promoted through irradiation of the material by electron beam bombardment. Within manufacturing processes, the electron beam must be of sufficient energy and intensity so that a sufficient number of electrons penetrate sufficiently into the material to provide the desired radiation treatment. Providing a controlled radiation dosage circumferentially to an outer surface of an elongated workpiece, such as a continuous strand or tube, has resulted in advantageous materials properties, such as differentially cross-linked polymer of the type described in commonly assigned U.S. Pat. No. 3,455,337.
However, providing a useful radiation circumferential dosage has been problematical with various approaches of the prior art, and providing a consistently uniform circumferential dosage has heretofore proven to be very difficult to achieve as an efficient or effective industrial process.
Electron beams used for irradiating materials such as small diameter wire and cable insulation, plastic tubes, tapes, filaments, strands and the like (hereinafter individually and collectively referred to as "strands") have required high accelerating voltages of up to one million volts, or more. Apparatus for generating electron beams for irradiation is typically large, and is typically contained in a special room or enclosure, and the irradiation process is carried out in a vault providing sufficient shielding so that radiation hazards are contained to the product being processed and are not presented to operating personnel.
In the case of irradiation of strands, one prior approach has been manually to thread the strand supplied from a feed reel about sets of sheaves in multiple loops, and then on to a take-up reel where the irradiated strand is coiled. The sheaves are contained in the vault adjacent to a thin metal foil exposure window of a scanned high energy electron beam accelerator. The multiple loops enable the strand to be irradiated during multiple passes through the exposure window. The amount and uniformity of irradiation has depended upon the scan rate and the travel rate of the strand, and upon an assumption that the strand rotates as it loops over the sheaves and therefore exposes the entirety of the surface thereof to irradiation from the scanned beam. This assumption is refuted somewhat by the fact that the strand takes a particular set when first loaded onto a takeup reel following the extrusion process, and this set is maintained during the subsequent irradiation process despite multiple loops of the strand about the sheaves and multiple loops of the strand passing by the exposure window.
Since irradiation of the looped strand has not been uniform, one prior proposal has been to provide permanent magnets below the sheaves under the exposure window. The magnets establish a fixed magnetic field of sufficient magnitude to reverse the direction of some portion of the electrons and return them to the sides of the looped strand opposite to the electron gun direction of incidence. While this approach has resulted in some improvement in the uniformity of irradiation, the irradiation process has remained cumbersome, and consequently off-line with respect to the extrusion process producing the strand.
Another approach, known in the art as the "toroid" approach, has been advanced. In this "single pass" approach, a window has been provided at the axial center of the apparatus. The window defines an orifice or passageway for the strand. A toroidal vacuum chamber radially surrounds the window; and a circular electron gun apparatus formed at the periphery of the toroidal chamber emits and accelerates electrons toward the window from e.g. 360 degrees. While this concept proposed a single-pass irradiation process, it has not yet been proven to be commercially practical or successful.
One other one-pass approach has been to dispose e.g. three electron guns about a coaxial window with 120 degree displacement from gun to gun. However, this approach is even more costly and cumbersome than the single swept-beam approach.
Despite the prior art approaches, a fundamental problem has been the unsolved need for a method and apparatus for maximizing radiation intensity and uniformity while at the same time exposing the strand to the radiation through a small window or port which is situated as closely to the strand as possible.
Thus, a hitherto unsolved need has arisen for a high energy particle beam generator enabling effective and more uniform irradiation of a strand which overcomes the limitations and drawbacks of the prior art.